I am going with the 1988 Andre Agassi when he broke onto the scene. That guy changed tennis. I saw it. I didn't like him at the time because of his brashness, but I like him now and have to give him credit. He used to wipe the court with people with that forehand that year. It was amazing. Plus, he wore crazy clothes and the hair (wig) spoke for itself. He was amazing, even to his detractors. I will also add the volatile Marat Safin and Goran Ivanisevic. You never knew if you were going to get unbelievably brilliant tennis or crazed racquet breaking with those two. It was almost soap opera like........ (How could we forget McEnroe back in the day?!?!?!?)